The man accused of faking sign interpretation while standing alongside world leaders like US President Barack Obama at Nelson Mandela's memorial service says he hallucinated that angels were entering the stadium, has schizophrenia and has been violent in the past.

Thamsanqa Jantjie said in a 45-minute interview with The Associated Press that his hallucinations began while he was interpreting and that he tried not to panic because there were "armed policemen around me". He added that he was once hospitalised in a mental health facility for more than one and a half years.

The statements by Jantjie also raise serious security issues for Obama, other heads of state and United Nations Secretary-General Ban Ki-moon who stood next to Jantjie as they made speeches at FNB Stadium in Soweto, Johannesburg's famed black township. The ceremony honoured Mandela, the anti-apartheid icon and former president who died on December 5.

A South African deputy Cabinet minister, Hendrietta Bogopane-Zulu, later held a news conference to announce that "a mistake happened" in the hiring of Jantjie. However, many questions remain, including who in the government hired the company that contracted Jantjie, how much money the government paid the company and Jantjie's own involvement with the company - and even whether it really exists.

AP journalists who visited the address of the company that Jantjie provided found a different company there, whose managers said they knew nothing about SA Interpreters. A woman who answered the phone at a number that Jantjie provided confirmed that she worked at the company that hired him for the memorial service but declined comment and hung up.

Government officials said they have tried to track down the company that provided Jantjie but the owners "have vanished into thin air", said Bogopane-Zulu, deputy minister of Women, Children and People with Disabilities.

She apologised to deaf people around the world who were offended by Jantjie's incomprehensible signing and said an investigation is under way to determine how Jantjie was hired and what vetting process, if any, he underwent for his security clearance.

The deputy minister said the translation company offered sub-standard services and the rate they purportedly paid the translator, US$77 (NZ$93) a day, is far below the usual rate of up to US$164 an hour.

Ordinarily, sign language interpreters in South Africa are switched every 20 minutes to maintain their concentration levels, she said. Jantjie was on the stage for the entire service that lasted more than four hours.

The deputy minister declined to say who in South Africa's government was responsible for contracting the company that provided the bogus translator, or how those rules were flouted.

"It's an interdepartmental responsibility," she said. "We are trying to establish what happened."

Jantjie insisted in the AP interview that he was doing proper sign-language interpretation of the speeches of world leaders. But he also apologised for his performance that has been dismissed by many sign-language experts as gibberish.

"I would like to tell everybody that if I've offended anyone, please, forgive me," Jantjie said in his tidy cement house outfitted with a big-screen TV and with two late-model cars in the carport on the outskirts of Soweto. "But what I was doing, I was doing what I believe is my calling. I was doing what I believe makes a difference."

"What happened that day, I see angels come to the stadium ... I start realising that the problem is here. And the problem, I don't know the attack of this problem, how will it comes. Sometimes I react violent on that place. Sometimes I will see things that chase me," Jantjie said.

"I was in a very difficult position," he added. "And remember those people, the president and everyone, they were armed, there was armed police around me. If I start panicking I'll start being a problem. I have to deal with this in a manner so that I mustn't embarrass my country."

Asked how often he had become violent, he said "a lot" while declining to provide details.

Jantjie said he was due on the day of the ceremony to get a regular six-month mental health checkup to determine whether the medication he takes was working, whether it needed to be changed or whether he needed to be kept at a mental health facility for treatment.

He said he did not tell the company that contracted him for the event that he was due for the checkup, but said an owner of SA Interpreters in Johannesburg was aware of his condition - which he says goes back nine years already.

A medical expert with University College London cautioned that Jantjie's unusual sign language didn't look like it was caused by schizophrenia or another psychosis.

"The disruption of sign language in people with schizophrenia takes many forms but this does not look like anything I have seen in signers with psychosis," said Jo Atkinson, who is a clinical psychologist and researcher at the Centre for Deafness, Cognition and Language.

He added: "There were many features of (his) signing that do not chime with the typical presentation of disordered signing caused by a psychotic episode."

Jantjie said he is officially classified as disabled by the government because of his schizophrenia, and that his longest period in a mental institution was in 2006, when he had a stay of one year and seven months.

Jantjie said he received one year of sign language interpretation training at a school in Cape Town. He said he has previously interpreted at many events without anyone complaining. The Deaf Federation of South Africa said it filed a complaint about his interpreting last year at an appearance that included President Jacob Zuma and other top governing African National Congress members.

The AP showed Jantjie video footage of him interpreting on stage at the Mandela memorial service.

"I don't remember any of this at all," he said.

Oscar-winning deaf US actress Marlee Matlin found a positive aspect to the whole incident, saying that it had made sign language a top news story across the world. "Sign went global today," she wrote on her Twitter account. "Wow thank you South Africa," she wrote in a second tweet.